[American Journal of Science, Vol. 308, February, 2008, P. 130 –166, DOI 10.2475/02.2008.02]
36
Cl DATING OF THE CLASSIC PLEISTOCENE GLACIAL RECORD IN
THE NORTHEASTERN CASCADE RANGE, WASHINGTON
STEPHEN C. PORTER† and TERRY W. SWANSON
ABSTRACT. Surface-exposure ages of granodioritic and gneissic boulders on seven
successive moraines record the Late Pleistocene history of a major glacier system in the
northeastern Cascade Range of Washington. The youngest moraines are boulder-rich
and sharp-crested, but successively older moraines have lower boulder frequencies
and are more degraded. Seventy-six 36Cl dates for the moraines cluster in groups
having mean ages (ⴞ 1␴) of 12,500 ⴞ 500, 13,300 ⴞ 800, 16,100 ⴞ 1100, 19,100 ⴞ
3000, 70,900 ⴞ 1500, 93,100 ⴞ 2600, and 105,400 ⴞ 2200 years; a still-older, highly
weathered and eroded moraine is undated, but likely is at least 165,000 years old. The
moraine ages and relative extent of the Icicle Creek glacier correspond closely to the
ages and relative amplitude of July insolation minima at 47.5° N latitude. They also are
in accord with ages for glaciations in the European Alps that were inferred by Milutin
Milankovitch and colleagues in the 1920s based on calculations of summer insolation.
Evidence that the greatest Late Pleistocene glacier advance occurred at the onset of the
last glaciation (during marine isotope substage 5d) is consistent with a cooling climate
and major glacier advances at that time elsewhere in North America and the Eurasian
Arctic, with a dramatic increase in global ice volume, and with the shifting track of the
jet stream over northwestern United States.
introduction
Temperate mountain glaciers and their deposits are important sources of Quaternary paleoenvironmental information, but determining the chronology of most glaciated alpine regions has been a continuing challenge because datable materials in or
associated with glacial deposits (organic matter, tephra, lavas) are often sparse or
lacking. Chronologies of Pleistocene alpine glaciation in the American West, based
largely on relative-age criteria, generally have been regarded as approximate or
provisional.
Within the past decade and a half, surface-exposure dating has been applied to
alpine glacial sequences in the western United States (for example, Phillips and others,
1990; Gosse and others, 1995; Licciardi and others, 2001). Ages of exposed moraine
boulders and associated bedrock surfaces can now be obtained routinely using one or
more cosmogenic isotopes (3He, 10Be, 26Al, 36Cl) and are providing new details and
insights about the chronology and dynamics of Pleistocene glaciers.
The upper Wenatchee River drainage basin on the east flank of the North Cascade
Range (fig. 1) has been the focus of several glacial-geologic studies. A sequence of
alpine moraines near the junction of the Wenatchee River and Icicle Creek provides
evidence of multiple advances of a large east-flowing Cascade glacier system. Until
recently, the only age control for these moraines and for those in most other Cascade
valleys was relative-age measurements using a variety of weathering criteria (Porter,
1976), and minimum limiting ages for some latest Pleistocene moraines based on lateand postglacial tephra layers (Porter, 1969, 1978; Crandell and Miller, 1974). Tentative
ages were assigned to the moraines based on inferred correlation with the standard
Midwestern North American ice-sheet succession (Sibrava and others, 1986) and with
marine oxygen-isotope stages (Martinson and others, 1987). Inherent in these correlations was the tacit assumption that times of greatest alpine-glacier advance correlated
Department of Earth and Space Sciences and Quaternary Research Center, University of Washington,
Seattle, Washington 98195-1310, USA
†
Corresponding author: E-mail address: [email protected]
130
S. C. Porter and T. W. Swanson
131
Fig. 1. Map of southern part of the northeastern Cascade Range showing location of Icicle Creek and
other geographic features mentioned in the text.
with times of maximum global ice volume inferred from the marine oxygen-isotope
record. Because ice-sheet maxima presumably coincided with the culminations of
glacial isotope stages (MIS stages 2, 4, 6. . .), mountain glaciers also were thought to
have reached maxima at these times. It also was generally assumed that the relative
magnitude of the mountain glacier advances matched that of the ice-volume maxima.
We undertook a restudy of the glacial history of the Icicle Creek glacier system
with the objectives of obtaining a cosmogenic-isotope chronology of the moraine
sequence, comparing this chronology with that of the adjacent Cordilleran Ice Sheet,
and assessing its relationship to changing Pleistocene climate. It involved additional
detailed mapping, resulting in further elaboration of the glacial record, and sampling
moraines for dating by the 36Cl method. Seventy-six dates provide a chronology for the
Late Pleistocene glacial succession and indicate that some previous assumptions
regarding correlations with the global ice-volume record likely are in error. Whereas
the new chronology shows that the alpine advances generally occurred at times of
ice-sheet advance, the relative magnitudes of alpine and ice-sheet advances were not
comparable.
earlier studies
Nearly seventy years ago, Page (1939) assigned the glacial deposits of the Icicle
Creek and adjacent drainages to three successively less-extensive glaciations, naming
them after Peshastin, Leavenworth, and Mount Stuart (fig. 1; table 1). He described
poorly exposed Peshastin drift as intensely weathered till and outwash gravel in which
granodiorite boulders up to a meter in diameter are weathered to their core. He
132
S. C. Porter and T. W. Swanson—36Cl dating of the classic Pleistocene glacial record
Table 1
Evolution of Icicle Creek glacial nomenclature
Page (1939)
Porter (1969)
Waitt (1977)
This study
Abbreviations
Stuart
Leavenworth IV
Leavenworth V
Rat Creek I and II
RCI, RCII
Leavenworth
Leavenworth I-III Leavenworth I-IV Leavenworth I and II
LI, LII
Not recognized
Not recognized
Not recognized
Mountain Home
MH
Not recognized
Not recognized
Not recognized
Pre-Mountain Home
pMH
Peshastin
Chumstick
Mountain Home
Peshastin
P
Not recognized
Peshastin
Boundary Butte
Boundary Butte
BB
mapped relatively unweathered Leavenworth drift forming a conspicuous lateral
moraine traceable for 5 km along the western flank of Boundary Butte Ridge (fig. 1)
and noted its resemblance to “a railroad embankment of gigantic proportions.”
During his “Stuart glaciation,” glaciers built conspicuous end moraines in Icicle Creek
valley and its southern tributaries. One at the mouth of Rat Creek he described as a
“perfect horseshoe” moraine and noted that it closely resembles Leavenworth deposits
downvalley.
Porter (1969) subsequently reported relative-age data for the Leavenworth and
two older drifts (Chumstick and Peshastin), subdivided the Leavenworth stage into
four substages, with the Rat Creek moraine of Page’s “Stuart glaciation” representing
the youngest (late-glacial) Leavenworth advance.
Waitt (1977) proposed additional changes to the nomenclature, redefining the
Leavenworth Drift to include five subunits (I-V), the youngest of which was equivalent
to Page’s Rat Creek moraines. He further proposed that Page’s Peshastin drift be
renamed Mountain Home Drift, and that the oldest (included by Page in his Peshastin
drift) be renamed Boundary Butte Drift.
revised moraine sequence
Our study has led to further refinement and revision of the Icicle Creek moraine
sequence. We retain Waitt’s designation Boundary Butte for the highest, oldest lateral
moraine and also retain the name Peshastin for the prominent moraine downslope
from it. We have identified a previously unrecognized lateral moraine between the
outermost Leavenworth lateral moraine and the Peshastin moraine, and have traced it
discontinuously for 5 km along the western slope of Boundary Butte Ridge. For most of
its length, the moraine lies above and nearly parallels Mountain Home Road, and we
now use the name Mountain Home for this post-Peshastin, pre-Leavenworth moraine
and the glacier advance during which it was built.
Two short lateral moraine segments lie outside the Mountain Home moraine at
the northern end of Boundary Butte Ridge. Both have subdued crests and few surface
boulders. We think that the outer of these may be of Peshastin age, as discussed below.
The inner we designate the pre-Mountain Home moraine.
We recognize only two Leavenworth moraine systems, Leavenworth I (older) and
Leavenworth II. The bouldery lateral moraines parallel one another along the west
slope of Boundary Butte Ridge, then descend steeply at the north end of the ridge
toward two corresponding end moraines on the valley floor. The inner moraine passes
obliquely through Leavenworth, where it has been extensively modified by human
activity. The outer is crossed by the Wenatchee River immediately south of its junction
with Chumstick Creek, and is best preserved south of the river.
in the northeastern Cascade Range, Washington
133
We also have mapped two nested moraines at the mouth of Rat Creek (Stuart
moraines of Page, 1939, and Leavenworth V moraines of Waitt, 1977) and named them
Rat Creek I (older) and Rat Creek II. Similar paired moraines occur in other tributary
valleys of Icicle Creek but these have not yet been studied in detail or dated.
previous age estimates
Page (1939) speculated that the Leavenworth deposits undoubtedly were ”equivalent to all or to part of the Wisconsin stage.” Porter (1969) inferred that the
Leavenworth drift was of late Wisconsin age, the intermediate drift was of early
Wisconsin age, and Peshastin drift was of pre-Wisconsin age. Waitt (1977) subsequently
estimated that the Leavenworth moraines are approximately 11,500 to 18,000 years
old, the Mountain Home Drift (Peshastin drift of this report) is about 130,000 to
140,000 years old, and Boundary Butte Drift is about 700,000 to 850,000 years old.
Waitt and others (1982) described evidence of a possible early Holocene glacier
advance in the Enchantment Lakes area at the crest of the Stuart Range, which forms
the southern headwaters of the Icicle Creek drainage (fig. 1). They noted that Glacier
Peak tephra layer G (Porter, 1978) can be traced to the outer edge of cirque moraines
that lie far above the Rat Creek moraines. This implies that the Rat Creek advance must
be older than 11,100 to 11,300 14C years, the likely age of the tephra (Foit and others,
1993). They concluded that the Rat Creek advance might be “as old as 13,000 yr B.P., if
not a little older.”
icicle creek glacier
During the greatest Leavenworth advance, the Icicle Creek glacier flowed as far as
46 km from cirques along the Cascade crest (fig. 2). The glacier system was strongly
asymmetrical. A dozen steep tributary ice streams, 2- to 4-km long, flowed from an ice
cap on the crest of Icicle Ridge (1980⫺2130 m altitude). An equal number of larger
southern tributary glaciers (3⫺13 km long) flowed from cirques (2135⫺2865 m)
along the crest of the Stuart Range. During the Mountain Home and Peshastin
advances the glacier probably terminated about 1 and 5 km, respectively, beyond the
Leavenworth terminal moraine. During the latter advance the glacier overflowed
(toward the east) almost the entire northern end of Boundary Butte Ridge. The
average surface gradient of the main ice stream was about 24 m/km; that of the longest
southern tributaries was 120 m/km. Owing to the strong climatic gradient across the
Cascades, the equilibrium line rose eastward across the glacier and its tributaries at
about 12 m/km (fig. 2). The ice is estimated to have been 380 m thick near the
equilibrium line in the main valley.
collection and analysis of samples for
36
36
Cl surface exposure dating
The Cl dating method was used in this study because the isotope production
rates were calculated using calibration samples from the adjacent Puget Lowland at the
same latitude and they were collected along an altitudinal gradient (sea level to
1600 m) that encompasses the altitude range of the Icicle Creek moraines (Swanson
and Caffee, 2001).
Boulders and smaller clasts comprising the moraines of the Icicle Creek glacier
are predominantly granodiorite. Many of the granodiorite rock samples we collected
have a high chloride content that produces 36Cl age versus erosion-rate profiles that
are relatively insensitive to the range of boulder-weathering rates inferred for the study
area (see below).
Samples were collected from boulders on the crests and upper slopes of moraines
and from glacially abraded bedrock. They were chipped off the top surface of the
largest moraine boulders (typically ⱖ2 m, but as much as 14 m) that appeared to have
remained undisturbed since deposition. Samples (generally 1⫺3 cm thick) were
134
S. C. Porter and T. W. Swanson—36Cl dating of the classic Pleistocene glacial record
Fig. 2. Topographic reconstruction of Icicle Creek glacier (contour interval 200 feet ⫽ 61 m) at the
time of the maximum Leavenworth I advance. The equilibrium line (bold line) crossed the surface of the
main glacier at about 4200 feet (1280 m), but because of its eastward-rising gradient, its altitude on tributary
ice streams lay at successively higher altitudes downvalley. Crests of major moraines in the lower reaches of
the valley include Boundary Butte (BB), Peshastin (P), Mountain Home (MH), and Leavenworth I and II (LI
and LII). The type Rat Creek moraines (RC) lie at the mouth of Rat Creek, the second major southern
tributary.
obtained, using a sledgehammer and mason’s chisel, from relatively flat surfaces (dip
angles ⬍15°) to minimize potential corrections related to the effect of surface
geometry on the incident cosmic ray flux. Topographic shielding of this flux was
estimated using an inclinometer, measured at 90° intervals around the horizon.
Shielding corrections for topography are generally less than 2 to 5 percent for all of the
measured samples and are incorporated into age calculations (Appendix).
Major-element analyses of rock samples were determined by X-ray fluorescence
(XRF) spectrometry, with an analytical uncertainty of ⬍2 percent and a detection limit
of 0.01 percent. Boron and gadolinium (elements with large cross-sectional areas for
neutrons) concentrations were measured by prompt gamma emission spectrometry,
with an analytical uncertainty of ⬍2 percent and a detection limit of 0.5 ppm. Uranium
and thorium contents were determined by neutron activation, with an analytical
uncertainty of ⬍2 percent and a detection limit of 0.5 ppm. Chemical analyses were
performed by XRAL Laboratories (Don Mills, Ontario, Canada). Total chlorine
content was determined by combination ion-selective electrode. At least five replicates
of each sample were measured to attain an analytical uncertainty of ⬍5 percent. In
cases of low chloride concentrations (⬍15 ppm), as many as 25 replicates of each
sample were run to attain an uncertainty of ⬍5 percent.
36
CL is produced in exposed terrestrial rocks mainly by cosmic-ray-induced
spallation reactions of K and Ca isotopes and by neutron activation of 35Cl. Muon
in the northeastern Cascade Range, Washington
135
capture by K and Ca isotopes becomes progressively more important in producing 36Cl
at low altitude and with increasing depth below the Earth’s surface. Noncosmogenic,
radiogenically produced 36Cl is generally in secular equilibrium and can be subtracted
from the total AMS-measured 36Cl to determine the cosmogenic component. The
remaining cosmogenic component can then be used to calculate surface exposure
ages using equations presented by Liu and others (1994).
A wet-chemical technique, modified from the procedure outlined by Zreda and
others (1991), was used to extract Cl from silicate rock. Samples were analyzed for 36Cl
by a tandem Van de Graaf accelerator at the Center for Accelerator Mass Spectrometry
(CAMS), Lawrence Livermore National Laboratory. Analytical uncertainties (1 ␴) of
the AMS measurements are typically 2 to 5 percent.
In calculating 36Cl ages, we used the production rates of Swanson and Caffee
(2001; Swanson, 2005), which were calibrated using the 14C-dated deglaciation history
of the northern Puget Lowland, Washington. Their calibration sites lie at a geomagnetic latitude similar to that of Icicle Creek and require little latitudinal scaling (table
2). Furthermore, because secular variation in the cosmic ray flux does not vary
significantly above 50° N latitude (Lal, 1991), the derived production rates are
applicable, with minor scaling, to surfaces that are both younger and older than the
surface from which they were originally calibrated. R. C. Finkel and T. W. Swanson
(unpublished data) have calculated 10Be ages for several of the Puget Lowland
calibration sites, and their calculated 10Be ages are consistent with calculated 36Cl ages
based on the Swanson and Caffee (2001) production rates.
We used production rates for calcium and potassium reactions of 87 ⫾ 7 atoms
36
Cl/g Ca/year and 228 ⫾ 23 atoms 36Cl/g K/year, respectively. The calculated
ground-level secondary neutron production rate in air, Pf (0), inferred from thermal
neutron absorption by 35Cl, is 762 ⫾ 28 neutrons/g air/year for samples with low water
content (1⫺2 wt. %). Production due to muon capture by 40Ca was estimated using a
value of ⬃5.0 ⫾ 1.0 atoms/g Ca/year that was derived empirically by Stone and others
(1998). These production rates were scaled to the altitude and latitude of the study
area using polynomials reported by Lal (1991). Due to variability in paleointensity of
the magnetic field over the integrated exposure history of a given sample, the
production rates used to calculate 36Cl ages have been scaled with respect to the
effective geomagnetic latitudes (table 2; Appendix) in the manner described by
Nishizumi and others (1989) and Clark and others (1995). Age calculations were made
using a boulder erosion rate of 2 mm/1000 years, midway between the inferred range
of boulder weathering scenarios of 1 to 3 mm/1000 years (see below). Ages were
calculated using a 36Cl exposure program (CHLOE) described by Phillips and Plummer (1996). Data used to calculate the 36Cl ages are given in the Appendix.
A scaling effect due to snow cover was included in the age calculation. In moist,
maritime mountain ranges like the Cascades, long-lasting seasonal snow can reduce
substantially the magnitude of the cosmic ray flux reaching the land surface. At Mount
Rainier, where record annual snowfalls of up to 30 m have been recorded, Holocene
moraine boulders typically remain snow-covered for 6 to 8 months of the year. Apostle
(2000, Apostle, ms 2004) sampled lahar boulders at 1735 to 1790 m altitude on the
southern slope of the volcano that have been exposed for the last 5000 to 5600 years
(Vallance and Scott, 1997) and reported that the boulders have 36Cl exposure ages half
that old, presumably the result of a thick and persistent snow cover during Neoglaciation (most of the last 5000 years).
The Pleistocene Icicle Creek moraines lie in the rain shadow of the eastern
Cascades. Annual snowfall at Leavenworth for AD 1948 to 2001 averaged 243 cm and
winter (DJF) snow depth averaged 17 cm (http://www.mthome.com/weather.html).
Snow-free conditions last from April to November. Snow cover at moraine sampling
Table 2
1
1
1
1
2
2
2
2
2
1
741
732
738
741
732
701
777
768
768
768
PESH 1-2
PESH 1-3
PESH 1-4
PESH 1-5
PESH 1-6
PESH 1-7
PESH 1-8
PESH 1-9
PESH 1-10
PESH 1-11
Sample
Thickness
(cm)
Altitude
(m)
Sample IDa
0.97
0.97
0.96
0.97
0.97
0.96
0.94
0.93
0.96
0.96
Shielding
Factor
668 ± 11
791 ± 12
871 ± 14
789 ± 15
678 ± 10
725 ± 14
1009 ± 15
756 ± 13
1009 ± 17
707 ± 23
Cl/Cl
± 1 sigma
(10-15 atoms)
36
104.7
108.2
114.4
108.1
108.7
116.1
112.8
100.4
112.0
Present
Latitute
(47.5° N)
113.4
Cl ages of Peshastin samples
36
156.8
112.6
101.7
105
110.9
105.3
143.2
144.1
147.0
144.0
143.7
147.5
109.5
105.5
127.3
141.0
97.5
108.6
101.7
105.0
110.9
105.3
105.5
112.6
109.5
97.5
108.6
97.3
98.5
106.1
99.0
99.8
98.7
101.0
112.1
101.8
103.8
112.3
108.3
103.8
106.8
93.8
110.5
91.9
104.3
101.8
103.8
115.9
104.9
107.0
117.0
112.3
96.1
115.1
Cl Exposure Age (1000 years)
Weathering Rate (mm/1000 years)
Effective
Latitudeb,c
0
1.1
2.2
3.3
4.4
(50.3°)
110
157.5
110.0
104.3
109.9
114.5
36
Example of effects of secular variation in paleomagnetic field intensity and boulder weathering rate on
136
S. C. Porter and T. W. Swanson—36Cl dating of the classic Pleistocene glacial record
in the northeastern Cascade Range, Washington
137
sites can be estimated using modern data from Mountain Home Lodge (altitude
613 m) above Leavenworth based on a 20 percent water equivalency. The estimate
includes uncertainty due to longer and shorter durations of seasonal snow at somewhat
higher and lower altitudes, respectively, than at the Mountain Home site. Reduced
precipitation during cold, dry glacial times (Whitlock and Bartlein, 1997) may have led
to only a thin seasonal snow cover on moraines, but lower melt-season temperatures
probably reduced ablation of the snow pack. Despite such uncertainties, corrections
for estimated snow cover at our sampling sites are generally in the range of 3 to 5
percent.
geologic constraints on moraine boulder ages
If all boulders on a moraine were freshly quarried, remained unweathered and
geomorphically stable since deposition, and lacked a prior history of exposure to
cosmic radiation, their isotopic ages should cluster in a tight group. However, outliers
in age populations show that this is not always the case, for generally a few boulder ages
lie statistically outside the primary age cluster. Such boulders can appear to be too old
or too young depending on their source and exposure history prior to and following a
glacier advance (fig. 3). In this study we consider a moraine boulder sample to be an
outlier in a sample population if it lies beyond one standard deviation of the
population mean. The exposure age of a moraine was obtained by calculating the
mean age of all samples lying within one standard deviation of the mean age of the
primary population cluster. However, the true age of the moraine is more likely
represented by the age of the oldest boulder that is not a statistical outlier (Putkonen
and Swanson, 2003). We assume that moraine ages obtained by the 36Cl dating of
exposed boulders represent minimum ages for moraine abandonment and stabilization.
Boulder Sources and Transport Paths
Large moraine boulders are overwhelmingly composed of coarse-grained granodiorite of the Mt. Stuart pluton. Metamorphic lithologies of the Chiwaukum Schist that
form Icicle Ridge are a minor component of the right-lateral moraines but are present
in the terminal moraines near Leavenworth and at Rat Creek. The likely origins of the
granitic boulders include (1) glacially plucked bedrock along the floor and margins of
Icicle Creek valley and its southern tributary valleys, (2) rockfalls from steep valley walls
onto the glacier surface, (3) incorporation of loose boulders from alluvium, colluvium,
and rockfall deposits on valley floors, and (4) reworking of older glacial deposits,
including terminal and lateral moraines (fig. 4).
Prior Exposure History
Moraines typically contain a mix of boulders with and without a prior history of
exposure to cosmic rays. The former usually appear as outliers in a population of
boulder ages (fig. 3). If averaged with the primary population, these outliers may
generate a mean age that is older than the age of moraine stabilization. The outermost
end moraine of any glaciation likely contains the largest component of such older
outliers. Reworked boulders can have various exposure histories, and may not form a
distinct older age cluster.
Many of the exposed large boulders on the Icicle Creek lateral moraines are
subangular to subrounded and probably represent fresh to variously weathered
rockfall debris that fell onto the glacier surface from adjacent valley walls. In the
glacier’s accumulation area, such rockfall debris would have been incorporated in the
glacier and transported subglacially and englacially toward the terminus (fig. 4).
Debris falling onto the glacier’s ablation area would have traveled mainly supraglacially
toward the ice margin and been deposited as a primary component of end moraines.
138
S. C. Porter and T. W. Swanson—36Cl dating of the classic Pleistocene glacial record
Fig. 3. Possible factors explaining the presence of older and younger outliers in a hypothetical
population of ten granitic moraine boulders and bedrock samples (see text).
Rockfalls reaching a glacier surface will include boulders with and without prior
exposure histories (fig. 4). If one assumes that one side of an equidimensional (cubic)
joint-bounded boulder had a long prior exposure history, then there is a 16 percent
chance (1 in 6) that the boulder, if deposited on a moraine crest, will lie with the
formerly exposed side upright (the side sampled for cosmogenic-isotope dating). For
glaciers subject to frequent rockfalls, a small percentage of boulders with a prior
exposure history can introduce anomalously old ages into the sample population.
Boulders plucked from deeply eroded (⬎2 m) valley-floor bedrock likely have had
little or no prior exposure, but those quarried from little-eroded surfaces may retain
inherited cosmogenic isotopes (fig. 4; Briner and Swanson, 1998) and appear anomalously old. As in the case of rockfall boulders, only one side of a large (⬎2 m) cubic
boulder plucked from bedrock is likely to have been exposed to cosmic radiation. All
sides of smaller boulders (⬍1 m) may retain some degree of prior exposure.
Because many old moraines contain boulders with some prior exposure history, it
is necessary to sample at least seven boulders on a moraine to identify these anomalous
in the northeastern Cascade Range, Washington
139
Fig. 4. Potential sources of boulders in Icicle Creek moraines
components (Putkonen and Swanson, 2003). In this study we sampled seven or more
boulders on most moraines, and commonly ten or more. With sample populations of
this size, outliers were identified more confidently and excluded when calculating a
moraine’s likely depositional age.
Moraine Degradation and Boulder Exhumation
As a moraine crest erodes and matrix is removed, exposed boulders may rotate,
shift, or roll. The top (sampled) surface will then have received less cosmic radiation
than will the top of a stable boulder and may appear as a young outlier in an age
population. A postglacial rockfall boulder deposited on a moraine would also have an
anomalously young age, unless its upper surface had a history of prior exposure.
When a boulder-rich lateral moraine is deposited, its distal slope typically attains
the angle of repose of the sediment, but the proximal slope may be much steeper.
Degradation of the moraine crest begins as the glacier withdraws, and rain, meltwater,
and mass-wasting processes begin to erode the matrix. An oversteepened proximal
slope will erode back until it stabilizes at a lower angle, in the process shifting the crest
in the distal direction. The crest is lowered and flattens as sediment is transferred
downslope (Porter, 1969), and originally buried boulders without prior exposure
emerge progressively at the surface (Hallet and Putkonen, 1994; Putkonen and
Swanson, 2003). Previously largely sheltered from cosmic radiation, these emerging
boulders will have younger exposure ages than surface boulders that have been
continuously exposed since moraine deposition, assuming the production pathway is
largely spallogenic.
140
S. C. Porter and T. W. Swanson—36Cl dating of the classic Pleistocene glacial record
Fig. 5. Excavation in Peshastin moraine near locality P-11 showing extensive granular disintegration of
2-m granite boulders in the weathering zone.
Granular Disintegration of Moraine Boulders
The coarse-grained granodiorite of the bouldery moraines along Icicle Creek and
Boundary Butte Ridge tends to weather mainly by granular disintegration, promoted
by oxidation of mafic minerals. On the Peshastin moraines, apparently little-weathered
surface boulders contrast with small buried boulders (ⱕ 0.5 m diameter) in the
weathering zone (1⫺3 m depth) that are entirely weathered to grus. Large subsurface
boulders in old moraines typically have a rind of grus tens of cm thick surrounding a
hard, little-weathered core (fig. 5). Large buried granitic boulders (1⫺2 m diameter
where exposed in section) in the weathering zone of the Boundary Butte drift are
completely weathered to grus. These observations imply that rates of decomposition
and granular disintegration in the weathering zone exceed those at the surface,
probably because greater subsurface moisture promotes more-intense chemical–
weathering reactions.
The long-term weathering of surface moraine boulders is illustrated by surface
boulder frequencies (SBF; the number of boulders within a given area along a moraine
crest) on successively older moraines (Porter, 1969). SBF is locally quite variable on the
Rat Creek and Leavenworth moraines because some very large boulders (ⱖ8 m
diameter) are present. Sectors with individual boulders covering more than 25 m2
therefore were avoided. Leavenworth moraines have an average SBF of 90⫺100/100
m2, the Mountain Home (previously informally called ‘Chumstick’) of 20/100 m2, and
the Peshastin moraine of ⬍1/100 m2. Surface boulders on the Boundary Butte
moraine are so rarely encountered that they do not provide a meaningful SBF value.
Spalling of Boulders
An additional important mechanism of surface boulder weathering is spalling
during major fires. The lower Wenatchee River basin is subject to frequent natural
in the northeastern Cascade Range, Washington
141
Fig. 6. Large granodiorite boulder (2 m tall) near crest of LI lateral moraine showing spalling around
sides resulting from intense 1993 forest fire in Icicle Creek drainage. Lichen-covered top of boulder (upper
right) is little modified. Carbonized tree (left) shows proximity of boulder to intense heat.
fires, for it lies in a zone of low mean annual precipitation (64 cm/year at Leavenworth) and high maximum summer temperatures (29°C). It supports a mixed coniferhardwood forest dominated by Ponderosa pine.
A large fire during 1993 swept across a sector of the Icicle Creek lateral moraines,
burning the forest and its understory and producing a noticeable affect on highstanding boulders. Those close to burning trees spalled on the side facing the most
intense heat (fig. 6). However, little or no fresh spalling was detected on the top
surfaces of large boulders (standing ⬎2 m high) in the fire zone. Possibly this was
because the flammable low understory vegetation generated the highest temperatures
near the boulders. These observations suggest that the primary effect of fires on some
large exposed granitic moraine boulders near Icicle Creek is the reduction of their
lateral dimensions; the top surfaces, where samples are taken for cosmogenic dating,
may be less affected. Nevertheless, during major fires all exposed boulder surfaces may
be subjected to spall-producing heat.
142
S. C. Porter and T. W. Swanson—36Cl dating of the classic Pleistocene glacial record
In assessing the role of fire in boulder weathering, the long-term fire history must
be considered. We have no independent measure of this but assume that fire
frequency was greatest under warm, dry interglacial and interstadial conditions.
During the late Quaternary, the frequency of large fires may have been highest during
the driest parts of the last interglaciation (about 125,000 to 117,000 years ago) and the
early to middle Holocene (about 13,000 to 4000 years ago) when climates were warmer
and drier than today in the eastern Cascades (Whitlock and Bartlein, 1997, table 2;
Whitlock and others, 2003). Paleoecological information from the Pacific Northwest
for 6000 cal years B.P. indicates that higher-than-present summer temperatures,
heightened drought, and drier soils and fuels, were all factors contributing to fire
frequencies higher than present (Whitlock and others, 2003). Thus, spalling events
related to an increase in major fires may be broadly episodic and linked to long-term
climate trends.
Degradation of Rock Surfaces and Long-Term Weathering Rates
An important parameter in the 36Cl age calculation is the estimated long-term
weathering rate of boulder and bedrock surfaces sampled for isotopic analysis. Such an
estimate is based on observed weathering features on surfaces whose age is known or
can be approximated. Glacially abraded granitic surfaces were sampled at several
places on the main valley floor (about 490⫺500 m altitude) to obtain limiting 36Cl ages
for deglaciation. Glacial grooves are reasonably common on these surfaces, but
striations (1⫺2 mm deep) and glacial polish are rarely observed, implying that most
were eradicated by postglacial weathering. Rock surfaces downvalley from the Rat
Creek moraines potentially have been exposed to weathering for about 15,000
years following deglaciation, suggesting that small-scale erosional features (up to
several mm deep) tend to be obliterated over this time interval (a weathering rate of
ⱖ0.15 mm/1000 years). Page (1939) and William A. Long (personal communication,
1980) reported polished and striated rock surfaces in the high southern tributary
valleys of Icicle Creek, which likely were deglaciated not long after deposition of the
inner Rat Creek moraine (about 13,000 years ago), and probably before about 11,500
years ago. Waitt and others (1982) reported scoured and striated quartz-diorite rock
surfaces in the Enchantment Lakes area (2300 m; fig. 1) beyond Neoglacial moraines,
but they also noted granular disintegration of granitic rocks that produced weathering
pits as deep as 10 cm, weathered joints, and pedestals. This degree of weathering may
well be related to lithology, for an estimated 20 percent of the local bedrock contains
easily weathered mafic minerals.
An average long-term weathering rate as high as 4⫺5-mm/1000 years is unlikely in
the case of continuously exposed granodiorite boulders on the Icicle Creek moraines.
Such a rate would mean that moraine crests about 100,000 years old would be nearly
devoid of boulders because 1 m or more of their diameter would be lost to weathering.
Nevertheless, a rate this high may apply in the subsurface weathering zone, where halos
of grus are ⱖ30 cm thick around granitic boulders of Peshastin age and ⱖ50 cm thick
around subsurface Boundary Butte boulders. A rate of 3 mm/1000 years, which would
reduce the diameter of a Leavenworth-age surface boulder by about 12 cm over 20,000
years, is probably an upper limit. Accordingly, we estimate that long-term average
weathering rates of granitic rock surfaces in this area that have not been subjected to
spalling fall between about 1 and 3 mm/1000 years (fig. 7). High rates might apply to
boulders in the weathering profile, especially during times of moist, warm climate. A
rate of ⱕ1 mm/1000 years may have characterized stadial intervals of the late
Pleistocene when cooler and drier conditions prevailed. For age calculations we have
used an intermediate value of 2 mm/1000 years, which is consistent with our SBF
measurements.
in the northeastern Cascade Range, Washington
143
Fig. 7. Influence of weathering rate on 36Cl age of moraine boulders. Inferred range of weathering rates
(1–3 mm/1000 years) for Icicle Creek moraines are delimited by dashed lines at these values; intermediate
value, (solid line) is used for age calculation. Possible ages for samples from Rat Creek (RC1–2),
Leavenworth (L1–23), Mountain Home (MH– 4), and Peshastin (P–2) within this range limit are shown
where curves intersect the weathering-rate values. Because our samples have a high Cl content thermal
neutron activation of 35Cl is the dominant production pathway. Unlike the spallogenic production pathways,
36
Cl produced by thermal neutron activation initially increases with depth and then decreases exponentially
below a depth of 18 cm (Liu and others, 1994). As a consequence, boulder ages are insensitive to erosion rate
between values of 1 and 4 mm/1000 years, especially in the case of older moraines. With higher weathering
rates (ⱖ5 mm/1000 years) degradation of weathered rock exposes new rock that extends below the depth of
the 36Cl production “pulse,” and calculated ages begin to rise.
icicle creek moraines and their
36
Cl ages
Because the pre-Rat Creek lateral moraines were deposited by Icicle Creek glaciers
having the same general flow path during each glaciation and transported sediment
from the same sources. The initial lithology and geometry of the successive moraines
probably were similar. However, the Rat Creek glacier terminated upvalley from
sedimentary rocks of the Swauk Formation that underlie all the older lateral moraines
on Boundary Butte Ridge, and its moraines are composed largely of granodioritic and
gneissic detritus.
Rat Creek Moraines
Two bouldery moraine loops cross lower Rat Creek (fig. 8) near its junction with
Icicle Creek at 550 m altitude. Granitic boulders on their crests reach diameters of 8 m
or more (fig. 9B). The outer moraine (Rat Creek I ⫽ RCI) has been truncated by Icicle
Creek, but the inner moraine (Rat Creek II ⫽ RCII) is a continuous bouldery ridge
that merges with the RCI left- and right-lateral moraines. The lateral moraines are
traceable to about 600 m altitude at the northeast end of the steep bedrock canyon of
Rat Creek. Similar moraines lie in comparable geographic positions in the other
southern tributary valleys of Icicle Creek (for example, Eightmile Creek; fig. 1).
Mazama tephra crops out behind the RCII moraine near the margin of a broad
144
S. C. Porter and T. W. Swanson—36Cl dating of the classic Pleistocene glacial record
Fig. 8. Maps of Rat Creek moraines (outer moraine ⫽ RCI, inner ⫽ RCII). (A) Localities where
moraine boulders were sampled. (B) 36Cl ages (1000 years) of sampled boulders.
depression filled with postglacial alluvium; its age of about 6800 14C years was the only
minimum limiting age for the moraines prior to the present study.
Seven samples for 36Cl dating were obtained from the RCI moraine and eleven
from the RCII moraine (fig. 8; Appendix). Ages (⫾ 1␴ AMS uncertainty) for the RCI
boulders range from 10,500 ⫾ 600 to 14,500 ⫾ 500 years and average 13,300 ⫾ 800
in the northeastern Cascade Range, Washington
145
Fig. 9. Boulders on lateral moraine crests showing relative frequency of granitic boulders and relative
degree of moraine degradation. (A) Little Ice Age right-lateral moraine of Nisqually Glacier, Mt. Rainier,
showing oversteepened proximal slope to right (60⫺70°) and distal slope to left (at angle of repose, about
34°); (B) Rat Creek II: the boulder is 6 m tall and 14 m long; (C) View along crest of Leavenworth I lateral
moraine. View is to northwest from Boundary Butte.
146
S. C. Porter and T. W. Swanson—36Cl dating of the classic Pleistocene glacial record
Fig. 9. (D) Boulders on crest of Mountain Home moraine; (E) Scattered boulders (0.5⫺2 m tall) on
Peshastin moraine; (F) Boulders (1⫺2 m tall) visible on distal flank and crest of Peshastin moraine (more
distant, lower) and absent on Boundary Butte moraine (closer, higher). View is to northwest from Boundary
Butte.
in the northeastern Cascade Range, Washington
147
years, excluding one outlier (RCI-1: 10,500 ⫾ 600 years). Ages for the RCII boulders
range from 11,600 ⫾ 900 to 16,000 ⫾ 500 years. Excluding one outlier (16,000 ⫾ 500
yr), their ages average age 12,500 ⫾ 500 years. If the assumed weathering rate (2
mm/1000 years) is reduced to 1 mm/1000 years, the mean age for the moraines
increases by 500 to 700 years. Regardless of which weathering rate is adopted, the mean
ages of the two moraines differ by less than 1000 years. The tight clustering of 36Cl ages
for each of the Rat Creek moraines likely is due to their small size, high boulder
frequency, and minimal degradation since deposition. It implies that their average
ages approximate the actual ages of moraine stabilization, which is consistent with
model results of Putkonen and Swanson (2003).
We obtained three samples (pRC⫺1, pRC⫺2, pRC⫺3) from glacially scoured
granitic bedrock and one (pRC⫺4) from a free-standing boulder on the floor of Icicle
Creek valley about 700 m east-southeast of the Rat Creek moraines. The bedrock
dates (18,600 ⫾ 1100 and 15,900 ⫾ 600 years) were included in the LII age
distribution, for this rock surface was progressively deglaciated following deposition of
the LII moraine. The oldest bedrock age (22,400 ⫾ 1000 years) falls within the range of
LI boulder ages and points to limited post-LI erosion at this site. The boulder has an
age of 13,600 ⫾ 600 years, similar to that of the RCI moraine boulders.
Leavenworth Moraines
Ice limits during the Leavenworth advances are defined by two terminal moraines
(LI and LII) in the town of Leavenworth and by corresponding lateral moraines on the
west slope of Boundary Butte Ridge (figs. 9C, 10, and 11). The Wenatchee River crosses
the bouldery LI terminal moraine about 650 m north of U. S. Highway 2. West of the
river the ice limit reaches the crest of a high bedrock hill north of Cascade High
School. The glacial limit was not identifiable beyond a lower bedrock hill just west of
the high hill; probably it lies beneath a broad, gently sloping alluvial fan built by
ephemeral streams that drain the southeast slope of Tumwater Mountain. The LII
terminal moraine crosses the Wenatchee River about 200 m north of the highway
bridge. In Leavenworth it forms a broad low ridge and a band of scattered large
granodiorite boulders that arcs toward the mouth of Tumwater Canyon 2 km to the
southwest (fig. 12A).
The LI and LII ice limits rise steeply up the northwest end of Boundary Butte
Ridge where they coincide with bouldery lateral moraines (figs. 10, 11, and 12B).
Mountain Home Road parallels the LII moraine for about 400 m before turning east to
pass across the LI moraine, which it follows southward for 5 km to where the two moraines
merge. The LI/LII moraine then continues south for another kilometer. Both moraine
crests are capped by concentrations of free-standing and partly buried granitic boulders,
many greater than 4 m in diameter and some exceeding 10 m. Road cuts show that the
moraine matrix is a poorly compacted diamicton of pebbly sand and silt.
About 2 km south of the north end of Boundary Butte Ridge the LI moraine
swings slightly east and is bordered by several small bouldery moraines. Their position
and geometry indicate that they predate the main LI ridge. Their similarity in boulder
weathering to that of the LI moraine and their low height suggest that the moraines
represent minor fluctuations of the LI ice margin on a scale similar to minor
fluctuations of alpine glaciers during the Little Ice Age.
Lateral moraines apparently are absent along the lower eastern slope of Icicle
Ridge. In contrast to the gentle west slope of Boundary Butte Ridge, the opposite valley
wall rises steeply, affording little chance for postglacial preservation of boulder-rich
ice-marginal sediments. Furthermore, the far-larger source areas of granitic boulders
in the southern tributaries of Icicle Creek compared to those entering from the north
means that sediment supply may have been much less to the glacier’s left-lateral
margin.
148
S. C. Porter and T. W. Swanson—36Cl dating of the classic Pleistocene glacial record
Fig. 10. Map of lower valley of Icicle Creek near its junction with the Wenatchee River showing
moraines of Icicle Creek glacier and sites where moraine boulders and bedrock surfaces were sampled.
Exposure ages of four boulders on the LI terminal moraine (LI⫺6, LI⫺7, LI⫺15,
LI⫺16) (figs. 10 and 12A) and seven boulders on the LI lateral moraine (LI⫺8
through LI–13 and LI–21 through LI–25 (figs. 10 and 12B) range from 11,700 ⫾ 800 to
in the northeastern Cascade Range, Washington
149
Fig. 11. Transect along Boundary Butte Ridge in lower Icicle Creek-Wenatchee River valley showing
positions of right-lateral moraine crests (bold lines; dotted where inferred) of Icicle Creek glacier. Triangles
are LI and LII terminal moraines.
45,100 ⫾ 1300 years and average 19,100 ⫾ 3000 years (excluding four outliers:
45,100 ⫾ 1300, 14,200 ⫾ 1300, 13,100 ⫾ 600, and 11,700 ⫾ 600 years; LI⫺9, LI⫺17,
LI⫺11, and LI–19, respectively. Five boulders along the crest and lower slope of the
steep bedrock hill at the northern edge of Leavenworth (L1-1, LI-2, LI-3, LI-4, and
LI–5) have ages ranging from 13,100 ⫾ 700 to 16,800 ⫾ 700 years (figs. 10 and 12A).
Excluding the youngest date, which we regard as an outlier, these boulders near the
limit of the LI advance have ages more consistent with the LII advance. However, they
lie well beyond and higher than the mapped LII ice limit. The anomalously young ages
are inferred to result from exhumation and (or) reorientation of boulders along the
southern side of the hillcrest when the Wenatchee River, occupying a large meltwater
channel along its southern base during the LII advance, undercut and steepened the
adjacent slope, thereby resetting the exposure age of the exhumed boulders. We
consequently have grouped these samples with the LII samples in calculating the age of
the LII ice limit.
Five ages for boulders of the LII lateral moraine (LII⫺1 through LII–5; figs. 10
and 12A) range from 12,600 ⫾ 800 to 21,200 ⫾ 600 years. Three outliers in this group
have ages of 12,600 ⫾ 600, 13,100 ⫾ 700, and 21,200 ⫾ 600 years (LII⫺3, LII⫺2, and
LII⫺4, respectively). The youngest two possibly are the result of boulder reorientation,
whereas the oldest is likely a reworked LI boulder. When combined with the exhumed
boulders (LI⫺1 through LI–5) north of Leavenworth (fig. 12A) the average age for LII
moraine stabilization is 16,100 ⫾ 1100 years.
Five samples were obtained from bedrock and erratic boulders on a rock knob
near the mouth of Tumwater Canyon (LII⫺6 through LII⫺10; fig. 10). They lie close
to the LII limit where it descends toward the canyon mouth. LII⫺6 and LII⫺7 were
mapped at the limit. Samples LII⫺8, LII⫺9, and LII⫺10 were collected just above the
LII limit, and are included in the sample average for the LI advance. One (LII⫺9) has
an LI age (19,000 ⫾ 2400 years) and one is considered a young outlier (11,700 ⫾ 800
years).
The 36Cl age distributions for the LII and LI moraines show greater scatter than
those for the Rat Creek moraines. Much of this scatter is best explained by postglacial
150
S. C. Porter and T. W. Swanson—36Cl dating of the classic Pleistocene glacial record
Fig. 12. Maps showing moraines of Icicle Creek glacier and ages of sampled boulders. Ages shown are in
thousands of 36Cl years. Open circles identify statistical outliers. (A) Leavenworth I and II terminal moraines
in and near Leavenworth, and possible Mountain Home moraine remnant. Large granitic boulders within
the city limits of Leavenworth, shown by small dots, form an arc that trends southwest toward the mouth of
Tumwater Canyon (fig. 10). Inferred ice limits shown by dashed lines.
degradation, rotation, and exhumation of boulders on the large Leavenworth lateral
moraines. Putkonen and Swanson’s (2003) model predicts increased age scatter for
large moraines like those of the Leavenworth advances, and it predicts that only one of
five boulder ages exceed 90 percent of the actual moraine age, with a 95 percent
probability. Thus, the stabilization ages of the LII and LI moraines may be closer to the
oldest boulder ages, excluding outliers, within their respective sample groups (17,000 ⫾
1000 and 24,700 ⫾ 1100 years, respectively) than to their calculated composite
averages (table 3).
Mountain Home Moraine
The Mountain Home lateral moraine is best preserved along the northern end of
Boundary Butte Ridge, about 1 km southeast of Leavenworth (figs. 10 and 12B). It
parallels Mountain Home Road for 2 km and rises upvalley from about 535 to 550 m
altitude (fig. 11). Through the lower 1 km of this reach, the moraine crest lies along
the west side of the main bedrock ridge. Farther south the glacier overtopped the ridge
crest to form lobes that terminated at altitudes of about 520 to 575 m in the upper
reaches of three small, unnamed valleys (figs. 11 and 12B). A segment of the moraine is
preserved on the slope of Boundary Butte Ridge 1.2 km north-northwest of Boundary
Butte, but elsewhere the Mountain Home ice limit is only inferred.
A subdued elongate north-northwest-trending ridge about 1 km north of Leavenworth and 500 m beyond the LI moraine may be a remnant of the Mountain Home
terminal moraine (figs. 10 and 12A). At the entrance to the canyon of Icicle Creek, the
Mountain Home limit probably coincides with a change in slope along the first
southern interfluve spur at about 730 m.
in the northeastern Cascade Range, Washington
151
Fig. 12. (B) Lateral moraines along the crest and northwestern slope of Boundary Butte ridge. Inferred
ice limits shown by dashed lines.
The crest of the lateral moraine is more subdued and less bouldery than the
Leavenworth lateral moraines (fig. 9D). Sandy gravelly drift exposed in a 2⫺m
excavation on the proximal slope near the moraine crest is weathered light-yellowish
brown. Most small (⬍10 cm) granitic stones in the soil profile are weathered to grus.
Erratic boulders along the crest have diameters of 1 to 3 m.
S. C. Porter and T. W. Swanson—36Cl dating of the classic Pleistocene glacial record
152
Fig. 12. (C) Sampled Peshastin (P) surface boulders nears the southern end of the main Peshastin
moraine and on small moraines near the southwestern end of Boundary Butte Ridge that cross the mouth of
a high tributary valley. Dashed line shows inferred Mountain Home ice limit.
Of ten boulders sampled along the crest of the Mountain Home moraine, we
regard one as a young outlier (29,000 ⫾ 1000 years; MH-7) and three as old outliers
(90,900 ⫾ 1400, 93,100 ⫾ 3100, and 118,100 ⫾ 2500 years); MH-5, MH-2, MH-8,
respectively). The remaining five dates range from 68,400 ⫾ 1800 to 72,200 ⫾ 1400
years and average 70,900 ⫾ 1500 years. A boulder near the east end of the moraine
remnant north of Leavenworth (fig. 12A) is older (77,500 ⫾ 2300 years; MH-10) than
the main cluster of MH boulder ages on Boundary Butte Ridge; its relationship to the
MH lateral moraine is uncertain.
Pre-Mountain Home, Post-Peshastin Moraine
Two subdued moraine segments, each about 500 m long, were mapped at the
north end of Boundary Butte Ridge (figs. 10 and 12B). The inner is 200 to 300 m east
of the crest of the Mountain Home moraine and the outer lies 250 to 400 m beyond it.
Table 3
Mean Age of Boulder Population and Age of Oldest Boulder
Moraine
Rat Creek II
Rat Creek I
Leavenworth II
Leavenworth I
Mountain Home
pre-Mountain Home
Peshastin
a
Excludes outliers
Population Meana
12,500 ± 500 (n=9)
13,300 ± 800 (n=6)
16,100 ± 1100 (n=11)
19,100 ± 3000 (n=17)
71,900 ± 1500 (n=5)
93,100 ± 2600 (n=3)
105,400 ± 2200 (n=8)
Oldest Date
13,500 ± 600
14,500 ± 500
17,000 ± 1000
24,700 ± 1100
72,200 ± 1400
94,900 ± 3100
112,800 ± 1700
in the northeastern Cascade Range, Washington
153
Both are broad-crested and have scattered surface boulders. Neither has previously
been described or mapped. Each is traceable to the abrupt steep bedrock slope that
descends to the floor of the Wenatchee valley. Their downslope trajectory is obscure;
probably each ice limit swings eastward to reach the main valley floor northeast of
Boundary Butte Ridge.
Two boulders on the inner moraine have ages of 91,200 ⫾ 3100 and 94,900 ⫾
3100 years (pMH-2, and pMH-1; average ⫽ 93,100 ⫾ 2600 years) and may date a glacier
advance between the Mountain Home and Peshastin advances. Two outliers of similar
age in the Mountain Home moraine (93,100 ⫾ 3100 and 90,900 ⫾ 1400 years) may be
reworked from this drift. The 109,300 ⫾ 3600-year age of a boulder on the outer
moraine (pMH-3) falls within the range of Peshastin boulder ages and suggests either
that this is a Peshastin lateral moraine segment or that a post-Peshastin advance
reworked and deposited a Peshastin-age boulder.
Peshastin Moraines
Page (1939) based his Peshastin stage on weathered drift along the Wenatchee
River valley near Peshastin (fig. 1) and inferred that a prominent lateral moraine
beyond the Leavenworth moraines on Boundary Butte Ridge was correlative with it.
Apparently included in his mapped Peshastin “till” was Mountain Home drift at the
northern end of Boundary Butte Ridge and the two small moraine segments east of it.
He also mapped Peshastin till in a broad zone on the northeastern slope of Boundary
Butte Ridge and north of the Wenatchee River, but we have been unable to confirm
this mapping. No unequivocal exposures of Peshastin drift were found in this zone,
now mostly covered by orchards. Like Page, we have been unable to identify a Peshastin
terminal moraine on the valley floor. Nevertheless, exposures of coarse weathered
outwash gravel and the gradient of the Peshastin lateral moraine are consistent with his
suggestion that the glacier probably terminated between Peshastin and Dryden (fig. 1).
The prominent Peshastin lateral moraine has a single linear crest traceable for 2
km along the southwestern slope of Boundary Butte Ridge (figs. 9E, 9F, 10, and 11).
Weathered granitic boulders up to several meters in diameter are scattered along its
flattened crest, the surface of which is modified by an unimproved forest road. The
moraine descends northeastward from about 780 to 750 m (15 m/km), and its distal
slope rises 13 to 25 m above the adjacent gentle western slope of Boundary Butte
Ridge. Farther north, the glacier overtopped the ridge and flowed into the Peshastin
Creek drainage (figs. 1, 10, and 11).
At its southwest end, the Peshastin moraine divides into three subdued ridges that
cross the lower end of a high, shallow unnamed tributary valley (figs. 10 and 12C). The
innermost and intermediate ridges appear to merge with the crest of the main lateral
moraine, but the outer ridge emerges from beneath the intermediate ridge.
Granitic boulders exposed in weathering profiles of the inner and intermediate
Peshastin moraines where they are crossed by Mountain Home Road are strongly
weathered (fig. 5). Boulders of more-resistant lithologies (basalt, quartzite) show only
modest weathering, and are not easily broken with a hammer blow.
Scarce surface boulders and a flattened cross profile of the lateral moraine point
to postdepositional weathering, degradation, and progressive exhumation of originally
buried boulders. To assess degradation, we measured five profiles across the moraine
crest and obtained average off-crest slope angles for the proximal and distal flanks
(32⫺34° and 15⫺30°, respectively; fig. 13). We assume that the cross profiles of all
moraines on Boundary Butte Ridge were similar at the time of deposition and
approximated those of recent moraines, for example lateral moraines of Nisqually
Glacier on Mt. Rainier (fig. 9A), and that subsequent degradation has modified the
steepness and width of the crestal region. Extrapolating the present distal and
proximal slope angles to their point of intersection above the moraine crest provides
154
S. C. Porter and T. W. Swanson—36Cl dating of the classic Pleistocene glacial record
Fig. 13. Diagrammatic profile (not to scale) across Peshastin lateral moraine showing effects of erosion
on exposure history of boulders. The initial profile is assumed to have been asymmetric, and similar to many
recent bouldery Cascade moraines (fig. 9A). Rapid raveling back of the oversteepened proximal face shifts
the crest progressively toward the distal side of the moraine until it achieves the angle of repose (32⫺34°),
leading to exhumation of buried boulders. Erosion of matrix and small clasts then results in lowering and
flattening of the crest. Projection of measured distal and proximal slopes suggests that the crest of the
Peshastin moraine at time of moraine formation was at least 2.7 m higher than now. Boulders a, b, and c,
each originally 2 m in diameter and with their tops continuously exposed since deposition, would be reduced
in size due to surface weathering during the past 100,000 years. Boulder a has moved downward as the crest
has lowered. Buried portions of boulders b, c, and d have a rind of grus surrounding a solid, less-weathered
core. Boulder d, originally buried within the moraine and now exhumed, has a shorter exposure history than
boulders b and c. Boulder e has entered the weathering zone as the crest lowered and has a halo of grus
surrounding its solid core. Boulders f and g lie beneath the weathering zone and remain relatively
unweathered. Boulder h, within the weathering profile and only 1 m in diameter, is weathered to the core.
Boulders that remained continuously at the surface, or below the weathering profile, would likely be less
weathered than those that spent a lengthy time in the weathering zone during moraine degradation. The
effects of fire have not been considered, but likely played a role in diminution of surface boulders, especially
during interglacial and interstadial intervals.
an approximate minimum height for the initial crest. For the main Peshastin lateral
moraine, this reconstructed crest height lies 2.1 to 3.2 m above the present degraded
crest. For the five profiles, the average postdepositional lowering of the crest is at least
2.7 m. Large boulders (ⱖ2 m diameter) buried close to the surface would have been
exposed relatively early as the loose matrix was eroded away.
Twelve boulders were sampled on the Peshastin moraines. Three samples were
obtained along the main lateral moraine crest (P-8 through P-10) and five from the
equivalent main crest in the high tributary valley (P⫺2, P⫺3, P⫺5, P⫺6, P⫺11). Four
others are young (P⫺4, P⫺12) and old (P⫺9, P⫺13) outliers. No suitable boulders
were found along the small outermost moraine, the crest of which has been bulldozed.
Eight of the twelve resulting exposure ages are tightly grouped, range from
102,500 ⫾ 1500 to 108,900 ⫾ 1700 years, and average 105,400 ⫾ 2200 years. Boulder
P-4 (93,700 ⫾ 1600) is a young outlier. Two older dates (112,700 ⫾ 1700 and 112,800 ⫾
1700 years; P⫺13 and P⫺9, respectively) are outliers, for they fall just outside the 1␴
range of the primary age cluster. If they were included in the cluster, the mean age
would increase from 105,400 ⫾ 2200 to 106,900 ⫾ 3700 years.
The innermost of the Peshastin moraines shown in figure 12C may either be of
late Peshastin or pre-Mountain Home age. Although we have mapped this moraine as
Peshastin, one date (P⫺12: 93,000 ⫾ 1500 years) is consistent with a pre-Mountain
home age, while the two older dates (P⫺7 and P13: 108,000 ⫾ 1600 and 112,700 ⫾
1700 years) fall in the range of Peshastin dates.
in the northeastern Cascade Range, Washington
155
Boundary Butte Moraine
The Boundary Butte moraine lies about 85 m below and 500 m northwest of the
top of Boundary Butte (965 m) (figs. 9F and 10). Its broad crest stands 70 to 100 m
higher than the adjacent Peshastin lateral moraine and descends northeast from about
875 m to 730 m over a distance of 2 km (fig. 11). Adjacent to Boundary Butte the crest
is broad, nearly flat, and has very few surface boulders (commonly only 20⫺30 cm of a
boulder is exposed). Granodiorite boulders as much as 2 m in diameter exposed in a
shallow road cut near the southwest end of the ridge are completely weathered to grus.
The effects of boulder weathering and moraine degradation have led to the
complete disappearance of all but a few small granitic boulders on the crest of
the Boundary Butte moraine. Some likely originated as much-larger boulders that
were weathered and exhumed as the moraine degraded. Surface-exposure ages for
such old moraines underestimate the moraine age. Our trial dating of several small,
likely exhumed boulders on the Boundary Butte moraine produced anomalously
young ages.
assessment of moraine ages
Of the 76 samples we dated, 10 are younger outliers and 10 are older (fig. 14).
Among the Leavenworth outliers twice as many are younger than are older, which may
reflect postglacial reorientation of boulders on unstable boulder-rich moraine crests.
Among Mountain Home ages, older outliers are more plentiful; all probably were
reworked from the pre-Mountain Home and Peshastin drifts.
Degraded Leavenworth and Rat Creek moraine crests often consist of loose piles
of large boulders with little or no exposed matrix. These bouldery crests appear
relatively stable, but they could become unstable during a large earthquake. The
northern Cascade Range is seismically active. Over thousands of years, occasional large
earthquakes may have caused moraine boulders to tumble or shift position, thereby
resetting the cosmogenic clock as fresh or less-impacted surfaces became exposed to
the full effect of cosmic radiation.
Significant lowering and flattening of moraine crests and exhumation of buried
boulders mainly affect the Mountain Home, Peshastin, and Boundary Butte moraines.
The mean age of Peshastin boulders (105,400 ⫾ 2200 years) probably underestimates
the depositional age of the moraine by at least several thousand years. The oldest
boulder dates, excluding outliers, are 112,800 ⫾ 1700 and 112,700 ⫾ 1700 years (table
3), which probably are closer to the time of moraine stabilization.
Because of long-term weathering and erosion, any pre-Boundary Butte glacial
deposits are likely to lack surface granitic boulders, although small clasts of moreresistant lithologies may still appear at the surface. Using our inferred range of average
long-term weathering rate (1 to 3 mm/1000 years), a 1-m boulder would be reduced to
grus in 500,000 to 165,000 years, respectively. At a rate of 5 mm/1000 years, complete
weathering of the boulder would take 100,000 years. The scarcity of boulders on the
Boundary Butte moraine (SBF ⬍1/100 m2) and grusified boulders at least 2 m in
diameter in the weathering zone lead us to conclude that the moraine is unlikely to be
less than about 165,000 years old nor more than 500,000 years (see below). This
moraine is the only mapped drift in the Icicle Creek sequence to be subjected to
weathering over at least two interglacial intervals (last and Holocene interglaciations)
during which a higher-than-average rate of boulder weathering may have prevailed.
In an early review of this paper, a referee expressed concern regarding the 36Cl
production rates used in this study and suggested that 10Be dates be determined for
several of the sampled boulders to compare with the ages obtained using the production rates of Swanson and Caffee (2001). Accordingly, 10Be ages were determined for
two Peshastin boulders (P⫺4: 113,300 ⫾ 10,800 years; P⫺7: 118,800 ⫾ 11,900 years)
156
S. C. Porter and T. W. Swanson—36Cl dating of the classic Pleistocene glacial record
Fig. 14. 36Cl ages ⫾ 1 ␴ (AMS uncertainty) of moraine boulders and bedrock sampled in the Icicle
Creek drainage. Statistical outliers are shown as crosses and were defined according to established protocol
discussed in the text. The length of the vertical line through each data point corresponds to the magnitude
of the AMS uncertainty. Mean moraine age ⫾ 1 ␴ of each sample population is shown at the top of the chart
and as a shaded area on the plot. Statistical outliers were excluded in calculating mean values. Also shown is
oldest date in each population. The MH date with an ‘x’ beside it is MH-10, the significance of which is not
clear. See text for discussion.
and for one Mountain Home boulder (M⫺6: 65,600 ⫾ 6200 years). These calculated
10
Be ages are consistent, within 1-sigma error ranges, with the corresponding mean
and oldest 36Cl ages reported in table 3 and the Appendix.
comparison of cascade glacier chronology with marine ␦18o, ice-sheet, and
deglacial variations, and with insolation trends
Based on our 36Cl chronology, Late Pleistocene fluctuations of the Icicle Creek
glacier can be compared with global ice-volume changes based on marine oxygen
isotopes, with advances of the nearby Cordilleran Ice Sheet, with the late-glacial
Younger Dryas event, and with insolation trends that may have controlled both climate
and the timing of mountain glacier expansions in the Pacific Northwest and the
European Alps.
Correlation with Marine Oxygen-Isotope Stages
At the outset of our work, we anticipated that dates for Leavenworth moraines
would fall within marine isotope stage (MIS) 2; our chronology bears this out (fig. 15).
The primary cluster of Mountain Home ages falls in MIS 4. Although we had surmised
that Peshastin drift likely was deposited during MIS 6, the tightly clustered 36Cl dates
indicate an MIS 5d age, about 20,000 years younger than the end of MIS 6. The dates
from the pre-Mountain Home moraine, and three (reworked?) outliers in the Mountain Home drift (90,900, 93,000, and 93,100 years), suggest an advance during MIS 5b.
in the northeastern Cascade Range, Washington
157
Fig. 15. Chronology and relative extent of Icicle Creek glacier advances compared with those of the
Puget Lobe of the Cordilleran Ice Sheet. These records are compared with the curve of global ice volume,
based on the standard marine oxygen-isotope record (Martinson and others, 1987), and with a curve of
summer (July) insolation for the latitude of the Icicle Creek glacier (47.5° N; data from M. F. Loutre,
personal communication, 2001). Whereas the relative magnitude of the ice sheet advances approximately
match those of the last three glacial marine-isotope stages (MIS 2, 4, and 6), the relative magnitudes of the
Icicle Creek glacier advances match those of prominent insolation minima (21,000, 70,000, 92,000, and
114,000 years ago) of the last 160,000 years.
Comparison with Cordilleran Ice Sheet Chronology
West of the Cascades, the chronology of all but the youngest advances of the Puget
Lobe of the Cordilleran Ice Sheet is not well constrained. During the last (Fraser)
glaciation, mountain glaciers coalesced and entered the Fraser Lowland of British
Columbia about 21,000 14C years (25,000 cal years) ago (Coquitlam advance; Hicock
and Armstrong, 1981; Booth and others, 2004). The Puget Lobe subsequently reached
its maximum extent about 14,000 14C years ago (16,900 cal yr ago) (Vashon advance;
Armstrong and others, 1965; Porter and Swanson, 1998) (fig. 15). The less-extensive
Possession advance likely culminated between 80,000 and 70,000 yr ago (Porter, 2004;
K. Troost, personal communication, 2006) during MIS 4. The Double Bluff advance
pre-dated last-interglacial deposits of the Whidbey Formation and likely dates to MIS 6.
Morainal deposits lying in a belt outside the Fraser drift limit in the southern lowland
may correlate with Double Bluff drift (Porter, 2004).
In figure 15, the timing and relative down-glacier extent of the Puget Lobe and
Icicle Creek glacier advances are compared with the marine oxygen-isotope stages. The
Coquitlam and Vashon maxima of the Fraser glaciation are close in age to the
Leavenworth I and II advances, respectively. The Possession advance may correlate
with the Mountain Home advance. Although the Double Bluff advance may have been
the greatest of the Puget Lobe, its likely MIS 6 age contrasts with the MIS 5d age of the
Peshastin, suggesting that these major ice sheet and alpine events were asynchronous.
Furthermore, the relative down-glacier extent of ice during successive advances was
dissimilar (fig. 15). Whereas the Possession glacier apparently was less extensive than
the Double Bluff and Fraser glaciers, the vertical separation of Icicle Creek lateral
moraines suggests a diminishing ice volume and downvalley extent of successively
younger alpine glaciers.
158
S. C. Porter and T. W. Swanson—36Cl dating of the classic Pleistocene glacial record
Correlation of Rat Creek Advances with European Younger Dryas
Kaufman and others (2004) have discussed whether the closely nested Rat Creek
moraines were built during the late-glacial Younger Dryas interval (12,900⫺11,600 cal
years B.P.). The mean 36Cl age for the Rat Creek I moraine (13,300 ⫾ 800 years)
predates the Younger Dryas interval, although the 1-␴ error range overlaps it. However,
the oldest date of a Rat Creek I boulder is 14,500 ⫾ 500 years, which predates the
Younger Dryas. The mean Rat Creek II age (12,500 ⫾ 500 years) falls within the range
of Younger Dryas time. These ages are consistent with another dated sequence in the
Cascades: the outer of two (Hyak-age) moraines at Snoqualmie Pass, south of Icicle
Creek, has a mean 36Cl age of 14,100 ⫾ 500 years (pre-Younger Dryas), while the
average age of the inner moraine (12,700 ⫾ 800 years) lies within the Younger Dryas
interval.
Comparison With Insolation Trends at 47.5° N Latitude
If the Icicle Creek record is compared with the long-term trends of July insolation
at this latitude, a close relationship is seen between the timing of alpine-glacier maxima
and insolation minima (fig. 15). Times of minimum July (summer) insolation
(21,000⫺22,000, 70,000, 92,000⫺93,000, and 114,000⫺115,000 years ago; data from
M. F. Loutre, personal communication, 2001) approximately match the mean and
oldest ages of the Icicle Creek glacier maxima (19,100 ⫾ 3000, 24,700 ⫾ 1100; 70,900 ⫾
1500, 72,200 ⫾ 1400; 93,100 ⫾ 2600, 94,900 ⫾ 3100; and 105,400 ⫾2200, 112,800 ⫾
1700 years ago; table 3). Less-prominent insolation minima (for example, 43,000 years
ago) are not represented by moraines, suggesting that glacier advances at such times
may have been more restricted in extent than the Rat Creek advance. The relative
extent of the major glacier advances, decreasing from Peshastin to Leavenworth,
parallels the relative amplitude of insolation minima from 114,000 to 21,000 years ago
(compare dashed lines in two right columns of fig. 15).
ages of the boundary butte and possible older advances
Although the Boundary Butte advance has not been dated, if the relationship
between insolation and glacier advances holds true prior to Peshastin time, the
Boundary Butte may correlate with one of the three next-older significant insolation
minima (186,000, 207,000, or 230,000 years) (fig. 16). The next-older minimum
(608,000 years ago) of equal amplitude is an unlikely candidate because weathering
over this long time interval would have destroyed boulders as large as 3.6 to 6 m in
diameter (3 and 5 mm/1000 yr, respectively) (fig. 16) and resulted in much greater
flattening of the Boundary Butte moraine crest than is observed. As many as 13
interglaciations since 608,000 years ago were comparable in intensity to MIS 1, and
these were times when weathering rates likely were higher than average.
Seventeen smaller-amplitude insolation minima occurred between 608,000 and
230,000 years ago during which glaciers probably expanded into the Icicle Creek
valley. Because most of these glaciers likely were confined to the middle and upper
reaches of the valley, morainal evidence of their existence has been obliterated.
However, each would have contributed to the long-term morphologic shaping of the
glacial valley.
insolation minima and alpine glacier maxima
A correspondence between solar radiation minima and glacial maxima in the
European Alps was proposed early in the last century (Imbrie and Imbrie, 1979, their
figs. 24 and 26). Using astronomical curves calculated by Serbian mathematician
Milutin Milankovitch (1920, 1941), Köppen and Wegener (1924) deduced that two
phases of the penultimate (Riss) glaciation (PGl 1, PGl 2) and three phases of the last
in the northeastern Cascade Range, Washington
159
Fig. 16. Trends of July insolation during the past million years at Icicle Creek (47.5° N latitude) (data
from M.-F. Loutre, personal communication, 2001). The last three prominent minima are correlated with
the last three major late Pleistocene Cascade glacier advances (L, MH, and P). The Boundary Butte advance,
if consistent with this pattern, may correlate with one of three strong minima at 187,000, 201,000, and
231,000 years ago. The next older minima of equal amplitude date to 608,000 and 968,000 years ago. Also
shown are values for boulder weathering at these times when weathering rates of 1, 3, and 5 mm/1000 years
are assumed (see text for discussion).
(Würm) glaciation (LGl 1, LGl 2, LGl 3) identified by Penck and Brückner (1909) were
correlative with summer insolation minima for 65°N latitude at 230,000 (PGl 1),
187,000 (PGl 2), 115,000 (LGl 1), 72,000 (LGl 2), and 25,000 (LGl 3) years ago,
respectively (updated version shown in Zeuner, 1958, his fig. 48) (fig. 17). They had no
way of testing this inferred chronology, for it was proposed decades before the
development of modern dating techniques that span the last several hundred thousand years.
The Icicle Creek glacial record supports this proposed Alpine chronology, for
inferred ages of the youngest three Alpine minima closely match the 36Cl mean and
maximum ages for major advances of the Icicle Creek glacier (fig. 17), and thus are
consistent with the Milankovitch astronomical chronology. Major glacier maxima in
both mountain ranges, each lying at 47 to 48°N latitude, correlate well with calculated
insolation minima at 25,000, 72,000, and 115,000 years ago. A minor insolation
minimum at 65° N that dates about 92,000 years (fig. 17) is a prominent minimum at
47.5° latitude in the Cascades (fig. 15), where it appears to correlate with the
pre-Mountain Home moraine (93,000 years).
160
S. C. Porter and T. W. Swanson—36Cl dating of the classic Pleistocene glacial record
Fig. 17. Proposed correlation of Alpine glaciations (PGl 1, PGl 2, LGl I, LGl 2, and LGl 3) with summer
insolation minima at 65° N latitude (in canonical units, C.U.), based on Milankovitch’s (1920, 1941)
calculations (from Zeuner, 1958, his fig. 48), compared with average ages (1000 years) and maximum ages
(italics) of 36Cl-dated moraines in the Cascade Range.
timing of the greatest late pleistocene glacier advance
The MIS 5d age of the Peshastin drift was not anticipated. Granitic boulders on
the degraded Peshastin moraine are strongly weathered, suggesting that the moraine
might predate the last interglaciation (MIS 5e). However, based on our 36Cl chronology, the greatest Late Pleistocene alpine-glacier advance occurred at the onset of the
last global glaciation (MIS 5d). This is consistent with Gillespie and Molnar’s (1995)
conclusion that the greatest Late Pleistocene alpine glacier advance predated the
maximum expansion of the continental ice sheets during MIS 2.
Although at first glance, the MIS 5d age for the Peshastin moraine may seem
anomalous, suggestive evidence elsewhere in the Northern Hemisphere also implies
glacier expansion during MIS 5d. For example, Phillips and others (1990) reported
36
Cl ages of 79,000 to 119,000 years for eight granite boulders on the Mono Basin
moraines of Bloody Canyon in the Sierra Nevada. When two outliers are excluded, the
mean age is 103,000 years. They state that if the effects of mineral weathering and
spalling are considered, the most likely age estimate for the moraines increases to
110,000 to 120,000 years. It nevertheless must be mentioned that this age assignment is
controversial because of disagreement over stratigraphic relationships and uncertainty
regarding cosmogenic-isotope production rates (A. R. Gillespie, in Kaufman and
others, 2004).
Two groups of moraines (C and D) built by the Bull Lake glacier on the east side
of the Wind River Range, Wyoming have yielded 36Cl ages of 95,000 to 125,000 and
95,000 to 115,000 years, respectively (Chadwick and others, 1997; Phillips and others,
1997). The dates from each moraine have considerable scatter, which may partly be
related to the small size of sampled boulders (1.5⫺ ⬍1 m), granular disintegration,
and spalling. Analytical uncertainties range from 3500 to 36,000 years and average ⫾
in the northeastern Cascade Range, Washington
161
15 percent. The youngest group (D) is 110,000 years old based on a combined 36Cl and
10
Be age model and is provisionally correlated with MIS 5d.
A major episode of glaciation recorded in sediments of Lake Baikal in eastern
Siberia occurred between 117,000 and 105,000 years ago (Karabanov and others,
1998). Glacial drift (Kormuzhikhantskaya moraine) within marine sediments in northwestern Siberia (62.5° N) is about 117,000 to 100,000 years old based on luminescence
dating (Karabanov and others, 1998). A short but significant episode of glacier growth
on Svalbard (‘Glaciation C’ at Kapp Ekholm) and in the Barents Sea occurred during
MIS 5d (Mangerud and others, 1998).
A cold episode in North Atlantic marine cores represented by peaks in the
percentage of polar foraminifera and ice-rafted detritus dates to about 107,000 to
103,000 years ago (McManus and others, 1994). Marine cores from the Pacific Ocean
show a substantial decrease in sea-surface temperature between the MIS 5e maximum
and the MIS 5d minimum. A rapid increase in global ice volume over this interval
equaled half that of full-glacial (MIS 2) time (Lea, 2004, fig. 6B; Shackleton, 2000,
fig. 4A).
The significant increase in global ice volume and local expansion of mountain
glaciers occurred close to the time of the major insolation minimum that culminated
114,000 to 115,000 years ago. The resulting reduction in summer temperature and
glacier ablation would have led to positive glacier mass balance and lowered ELAs,
causing glaciers to expand.
The Pacific Northwest’s strongly maritime climate is a significant factor in
regional glacier mass balance, for Cascade glaciers are nourished by winter storms
coming off the Pacific. Model simulations and pollen data suggest that during the last
glacial maximum (MIS 2) winter (January) storm tracks shifted well south of their
present positions, creating colder and drier conditions in the Pacific Northwest
(Kutzbach and others, 1993; Thompson and others, 1993). Subsequent deglacial
shrinkage of the Cordilleran Ice Sheet in western Canada allowed storm tracks to shift
northward, bringing enhanced snowfall to southwestern British Columbia and to
northwestern Washington as the Puget Lobe expanded to its full-glacial maximum
about 17,000 years ago..
Geologic evidence from northwestern Europe, Svalbard, and the Barents Sea
points to expansion of northern ice sheets in these areas during MIS 5d (Mangerud,
1991; Mangerud and others, 1998), but less than during MIS 2 and 4. In the Pacific
Northwest, evidence of an MIS 5d ice-sheet advance has not yet been documented, but
if an incipient ice sheet also began to form in southwestern Canada at that time,
westerly Pacific storms may have passed mainly south of it across the Washington
Cascades. As a result, winter snowfall might not have been reduced during the time
that Peshastin-age glaciers reached their greatest size under summer-cool, winter-moist
conditions.
acknowledgments
This research was supported in part by National Science Foundation grants
ATM-9009104 and 9632367. We are indebted to Robert Finkel, Marc Caffee, and the
Center for Accelerator Mass Spectrometry (Lawrence Livermore National Laboratory)
for their assistance with AMS measurements of 36Cl and 10Be isotopes. We also thank
Jason Briner, Daniel McCrumb, and David Mohler for laboratory and field assistance;
André Berger and Marie-France Loutre for insolation data; and William A. Long and
Richard B. Waitt for information and discussions arising from their experience with
and interest in the glacial history of the North Cascades. We are grateful to Richard B.
Waitt and John J. Clague for thoughtful and constructive reviews of the manuscript.
47.54
47.54
47.54
47.54
pRC-1
pRC-2
pRC-3
pRC-4
LII-1
LII-2
LII-3
LII-4
LII-5
LII-6
LII-7
LI-1
LI-2
LI-3
LI-4
LI-5
LI-6
LI-7
ICE 1-1(BR)
ICE 1-2 (BR)
ICE 1-3 (BR)
ICE 1-4 (ER)
LEAV 2-1
LEAV 2-2
LEAV 2-3
LEAV 2-4
LEAV 2-5
LTC-4 (BR)
LTC-5 (BR)
LEAV 1-1
LEAV 1-2
LEAV 1-3
LEAV 1-4
LEAV 1-5
LEAV 1A-1
LEAV 1A-2
47.60
47.60
47.60
47.60
47.60
47.60
47.60
47.59
47.59
47.58
47.58
47.58
47.58
47.58
47.55
47.55
47.55
47.55
47.55
47.55
47.55
RCI-1
RCI-2
RCI-3
RCI-4
RCI-5
RCI-6
RCI-7
RAT 1-1
RAT 1-2
RAT 1-3
RAT 1-4
RAT 1-5
RAT 1-6A
RAT 1-6B
49.3
50.9
48.9
49.3
49.0
49.0
49.0
49.0
49.0
49.3
49.3
49.3
49.3
49.3
48.3
48.3
48.3
48.3
47.4
47.9
47.9
47.9
47.9
47.9
47.9
120.64
120.64
120.66
120.66
120.66
120.66
120.66
120.68
120.68
120.65
120.65
120.65
120.65
120.65
120.74
120.74
120.74
120.74
120.76
120.76
120.76
120.76
120.76
120.76
120.76
2.7
2.7
2.7
2.7
2.7
2.7
2.7
415
421
402
402
402
402
402
2.7
2.7
2.7
2.7
2.7
2.7
2.7
494
494
488
488
488
360
360
2.7
2.7
2.7
2.7
2.7
2.7
2.7
2.7
2.7
2.7
2.7
2.7
2.7
2.7
2.7
2.7
2.7
2.7
2.7
2.7
2.7
2.7
1.0
1.0
1.0
1.0
1.0
1.0
1.0
1.0
1.0
1.0
1.0
1.0
1.0
1.0
2.0
1.0
1.0
1.0
1.0
1.0
1.0
1.0
1.0
1.0
1.0
1.0
1.0
1.0
1.0
1.0
1.0
2.0
2.0
2.0
2.0
2.0
0.94
0.96
0.97
0.97
0.97
0.97
0.97
0.92
0.92
0.94
0.94
0.94
0.94
0.94
0.87
0.87
0.87
0.87
0.92
0.92
0.92
0.92
0.92
0.92
0.92
0.92
0.92
0.92
0.92
0.92
0.92
0.92
0.92
0.92
0.92
0.92
Densityb Thickness Shielding
cm
All
gm/cm3
effects
562
562
562
562
573
573
573
588
558
549
549
555
555
555
555
555
555
555
555
555
562
562
47.55
47.55
47.55
47.55
47.55
47.55
47.55
47.55
47.55
47.55
47.55
RCII-1
RCII-2
RCII-3
RCII-4
RCII-5
RCII-6
RCII-7
RCII-8
RCII-9
RCII-10
RCII-12
RAT 2-1
RAT 2-2
RAT 2-3
RAT 2-4
RAT 2-5
RAT 2-6
RAT 2-7
RAT 2-8A
RAT 2-8B
RAT 2-9
RAT 2-11
120.76
120.76
120.76
120.76
120.76
120.76
120.76
120.76
120.76
120.76
120.76
Sample Latitude
Eff.
Longitude Altitude
Latitude
m
Number
°N
°W
(Map)
°Na
Sample
Number
(Field)
47.9
47.9
47.9
47.9
47.9
47.9
47.9
47.9
47.9
47.9
47.9
Appendix
16.4 ± 0.7
22.3 ± 1.3
16.8 ± 0.7
13.1 ± 0.7
17 ± 1.0
15 ± 0.7
15.4 ± 0.6
15.7 ± 1.0
16.1 ± 1.1
16 ± 0.8
14.9 ± 0.7
12.6 ± 0.6
21.2 ± 0.6
14.6 ± 0.5
22.4 ± 1.0
15.9 ± 0.6
18.6 ± 1.1
13.6 ± 0.6
10.5 ± 0.6
14.5 ± 0.5
13.3 ± 0.8
12.8 ± 0.8
13.8 ± 0.6
12.6 ± 0.5
12.5 ± 0.9
12.7 ± 0.5
11.6 ± 0.9
12.6 ± 0.6
13.5 ± 0.6
16.0 ± 0.5
12.0 ± 0.9
12.3 ± 0.9
12.4 ± 0.5
12.7 ± 0.7
12.3 ± 0.6
12.4 ± 0.7
Cl Age ±
1sc
36
73
108
108
59
78
59
75
75
81
82
68
68
141
91
107
83
128
62
54
87
67
64
66
74
66
61
59
76
58
85
57
66
64
59
55
61
16.90
16.90
16.60
15.10
15.60
14.70
14.90
14.70
14.60
15.20
15.00
15.60
15.00
17.40
17.40
17.30
15.60
15.60
15.20
15.50
15.20
15.10
15.60
14.20
14.60
15.20
15.40
56.60
58.80
56.50
60.60
61.00
54.30
51.60
54.30
54.10
57.60
58.10
58.20
60.00
57.70
55.50
57.60
58.90
61.00
59.50
55.30
54.80
59.60
56.30
54.50
56.90
57.30
59.20
0.19
0.29
0.19
0.15
0.11
0.06
0.04
0.06
0.15
0.04
0.09
0.07
0.11
0.18
0.08
0.20
0.16
0.08
0.17
0.07
0.11
0.07
0.11
0.10
0.09
0.17
0.16
0.07
0.08
0.17
0.09
0.08
5.55 15.40 57.60 0.17
3.81 17.10 55.50 0.19
64.10
58.40
55.30
55.00
62.80
4.44
5.48
14.90
16.70
17.30
15.80
14.70
2.42
3.18
4.16
4.78
2.66
5.80 15.40 58.20 0.19
6.57 13.30 58.00 0.07
4.17
4.10
3.16
3.83
2.88
7.17
6.86
7.17
5.78
3.95
3.61
3.44
4.68
4.59
4.04
4.78
4.37
4.09
4.78
3.68
4.10
5.31
4.08
4.46
4.70
5.35
4.27
4.58
5.45
5.04
5.48
4.97
3.40
2.92
4.68
4.79
5.57
4.77
4.77
4.74
4.67
4.24
4.96
5.61
4.97
5.28
3.72
3.95
4.04
3.95
4.42
3.76
3.89
5.64
5.80
3.61
5.74
5.75
5.30
4.34
4.83
0.94
0.94
1.81
1.57
0.96
1.57
1.91
1.45
1.27
0.92
1.26
1.80
1.74
1.89
0.65
0.39
0.07
0.90
1.18
1.29
1.44
1.23
3.95
0.89
0.95
1.65
1.58
2.32
1.27
1.58
1.23
1.08
1.15
1.14
1.60
1.43
5.76
5.82
4.03
4.90
6.27
5.58
4.07
5.88
5.62
6.05
5.24
5.23
4.45
4.45
6.45
6.82
6.45
5.72
4.95
5.06
4.85
5.24
6.45
6.33
6.53
5.05
5.43
4.23
5.13
4.97
5.31
5.34
5.04
5.16
5.16
5.21
0.72
0.68
0.45
0.59
0.73
0.65
0.61
0.70
0.75
0.76
0.68
0.62
0.63
0.55
0.69
0.67
0.69
0.79
0.56
0.60
0.58
0.66
0.67
0.66
0.70
0.77
0.62
0.79
0.58
0.58
0.80
0.64
0.60
0.54
0.73
0.57
6.03
5.41
6.30
5.11
5.37
6.37
5.43
5.55
5.75
6.40
5.27
0.10
0.10
0.06
0.07
0.10
0.09
0.06
0.10
0.12
0.10
0.08
0.08
0.07
0.07
0.13
0.13
0.15
0.12
6.88
6.05
4.03
5.18
6.55
6.09
4.63
6.66
7.45
6.22
6.04
5.81
5.22
4.81
7.93
7.67
8.93
7.86
0.08 4.99
0.10 5.50
0.08 5.50
0.09 5.69
0.11 17.40
0.09 5.72
0.11 6.43
0.10
0.08
0.09
0.08
0.08
0.10
0.08
0.09
0.09
0.10
0.10
432
316
163
391
476
885
424
436
313
385
673
700
228
241
574
279
157
497
674
330
626
592
538
401
462
722
917
307
754
528
636
577
748
1067
868
672
3.50
4.00
4.00
4.00
4.00
3.00
3.50
0.25
3.00
3.50
3.50
3.00
4.00
4.50
2.00
2.00
3.00
0.50
0.25
3.00
4.00
2.00
2.00
4.50
3.00
4.00
6.00
5.00
3.00
3.00
4.50
2.00
27.00 5.00
27.00 5.50
30.00
30.00
26.00
32.00
40.00
40.00 3.00
29.00 0.50
15.00
30.00
30.00
31.00
31.00
21.00
20.50
8.00
27.00
24.50
24.50
34.00
32.00
35.00
19.00
35.00
25.00
27.00
37.00
56.00
27.00
35.00
35.00
30.00
30.00
34.00
23.00
5.00
5.50
5.00
3.00
3.00
4.50
2.00
3.00
0.50
3.50
4.00
4.00
4.00
4.00
3.00
3.50
0.25
3.00
3.50
3.50
3.00
4.00
4.50
2.00
2.00
3.00
0.50
0.25
3.00
4.00
2.00
2.00
4.50
3.00
4.00
6.00
1.30
1.10
0.25
0.25
0.80
0.80
1.70
1.70
0.25
0.25
1.50
2.70
2.70
1.50
0.25
0.25
1.10
1.20
1.00
1.40
1.50
1.30
0.80
0.90
1.10
0.90
2.20
0.90
1.30
1.50
1.50
1.80
1.50
1.00
0.70
0.90
2.80
2.30
2.70
0.25
2.30
2.30
3.20
3.80
0.25
0.25
2.00
5.90
5.90
2.00
1.30
0.25
2.80
1.00
3.00
3.00
3.20
2.90
2.40
2.80
4.40
2.30
1.40
2.30
2.90
2.10
1.60
2.10
3.20
2.10
1.80
1.90
Gd
U
Th
Sm
B
Cl/Cl Na2O MgO Al2O3 SiO2 P2O5 K2O CaO TiO2 MnO Fe2O3 Cl
wt. % wt. % wt. % wt. % wt. % wt. % wt. % wt. % wt. % wt. % ppm ppm ppm ppm ppm ppm
36
Cl ages and geographic, outcrop, and chemical data for Icicle Creek samples
36
162
S. C. Porter and T. W. Swanson—36Cl dating of the classic Pleistocene glacial record
LI-15
LI-16
LII-8
LII-9
LII-10
LI-8
LI-9
LI-10
LI-11
LI-12
LI-13
LI-21
LI-22
LI-23
LI-24
LI-25
MH-1
MH-2
MH-3
MH-4
MH-5
MH-6
MH-7
MH-8
MH-9
MH-10
pMH-1
pMH-2
pMH-3
LTC-1 (BR)
LTC-2 (ER)
LTC-3 (ER)
LEAV 1B-1
LEAV 1B-2
LEAV 1B-3
LEAV 1B-4
LEAV 1B-5
LEAV 1B-6
PL 1-2
PL 1-3
PL 1-4
PL 1-5
PL 1-6
MH 1-1
MH 1-2
MH 1-3
MH 1-4
MH 1-5
MH 1-6
MH 1-7
MH 1-8
MH 1-9
MH 1-1T
MH 2-1
MH 2-2
MH 2-3
47.59
47.59
47.59
47.59
47.59
47.59
47.58
47.58
47.58
47.58
47.58
47.58
47.61
47.58
47.58
47.58
47.58
47.58
47.58
47.58
47.56
47.56
47.56
47.58
47.59
47.59
47.59
47.61
47.61
50.6
50.6
50.6
50.6
50.6
50.6
50.6
50.6
50.6
50.6
50.6
50.6
50.6
49.5
49.5
49.5
50.8
49.5
50.8
50.6
50.8
50.8
50.9
50.9
49
49.0
49.0
49.3
49.5
120.65
120.65
120.65
120.65
120.65
120.65
120.65
120.65
120.65
120.65
120.65
120.65
120.66
2.7
2.7
2.7
561
561
588
2.7
2.7
2.7
2.7
2.7
524
549
549
546
527
2.7
2.7
2.7
2.7
2.7
2.7
2.7
2.7
2.7
2.7
2.7
2.7
2.7
2.7
2.7
2.7
573
573
604
604
604
605
120.68
120.68
120.65
120.65
120.65
120.65
585
585
585
576
573
573
549
576
597
378
2.7
2.7
2.7
433
433
433
120.68
120.68
120.68
120.65
120.65
120.65
120.65
120.65
2.7
2.7
1.0
1.0
2.0
2.0
3.0
1.0
1.0
2.0
1.0
1.0
1.0
2.5
2.0
1.0
1.0
1.0
1.0
1.0
1.0
1.0
1.0
1.0
1.0
2.0
1.0
1.0
1.0
1.0
1.0
0.96
0.96
0.96
0.96
0.96
0.96
0.97
0.96
0.96
0.95
0.96
0.97
0.97
0.93
0.92
0.93
0.93
0.93
0.94
0.94
0.93
0.93
0.93
0.95
0.92
0.92
0.92
0.95
0.95
Densityb Thickness Shielding
gm/cm3
cm
All
effects
366
366
120.66
120.66
Sample Latitude
Longitude Altitude
Eff.
Latitude
m
Number
°N
°W
a
°N
(Map)
LEAV 1C-1
LEAV 1C-2
Sample
Number
(Field)
94.9 ± 3.1
91.2 ± 3.1
109.3 ± 3.6
70.9 ± 1.8
93.1 ± 3.1
71.4 ± 1.4
72.2 ± 1.4
90.9 ± 1.4
68.4 ± 1.8
29 ± 1.0
118.1 ± 2.5
71.8 ± 1.0
77.5 ± 2.3
15.8 ± 0.6
18 ± 0.6
19.2 ± 0.7
23.4 ± 0.7
17.8 ± 0.5
20.1 ± 0.8
45.1 ± 1.3
21.7 ± 0.9
13.1 ± 0.6
17.9 ± 1.1
24.7 ± 1.1
14.2 ± 1.3
19 ± 2.4
11.7 ± 0.8
15.3 ± 0.8
15.8 ± 1.1
870
639
867
736
868
470
650
789
421
220
732
643
520
66
96
93
101
153
116
334
123
110
106
182
71
113
69
94
63
4.20
3.73
4.25
3.99
3.88
3.91
4.16
4.17
4.09
3.99
2.58
4.17
3.29
4.30
3.79
3.71
4.04
4.17
15.70
14.60
15.70
15.60
15.60
15.90
15.30
13.00
15.90
16.20
16.50
16.00
15.80
16.40
15.90
62.10
65.00
57.80
64.40
63.90
62.50
63.10
55.30
61.10
51.80
61.80
57.01
56.30
58.50
63.20
0.14
0.06
0.07
0.09
0.07
0.06
0.06
0.37
0.17
0.21
0.15
0.17
0.18
0.17
0.09
0.23
0.15
0.18
0.06
0.04
0.12
2.51 15.40 65.30 0.05
6.89 16.40 54.00 0.05
2.79 15.50 63.30 0.07
3.52
2.70
5.49
2.51
2.62
3.31
3.19
9.71
3.43
8.34
3.70
5.01
5.91
4.27
2.75
57.30
62.40
59.70
63.70
61.20
63.30
4.09
2.64
4.66
2.79
3.58
2.62
4.71
4.82
3.74
3.91
4.08
4.51
17.20
15.80
15.70
14.90
16.30
16.90
3.60 14.70 63.00 0.06
3.29 16.60 61.40 0.04
6.36 15.20 55.10 0.22
2.70 14.70 62.80 0.07
3.55 15.90 60.70 0.06
3.78
4.05
3.61
3.91
4.34
1.44
0.52
1.36
0.93
2.13
0.70
1.72
1.55
0.86
1.65
0.47
1.77
0.36
1.55
1.18
1.81
1.34
1.93
0.98
1.42
1.64
1.13
0.75
1.31
1.44
1.16
0.64
1.30
1.42
4.95
7.64
5.17
5.79
4.48
6.86
4.80
4.91
6.04
5.16
6.34
5.35
8.39
5.54
6.98
6.47
6.42
4.58
6.03
4.64
5.84
5.17
5.99
5.26
5.20
5.59
7.44
4.83
5.68
0.46
0.97
0.50
0.58
0.49
0.69
0.49
0.50
0.65
0.53
0.67
0.59
1.00
0.61
0.75
0.63
0.67
0.56
0.87
0.49
0.65
0.47
0.60
0.47
0.50
0.55
0.78
0.47
0.61
0.07
0.13
0.07
0.09
0.06
0.13
0.06
0.07
0.09
0.08
0.16
0.09
0.14
0.08
0.11
0.12
0.10
0.07
0.08
0.07
0.08
0.07
0.09
0.06
0.07
0.09
0.12
0.60
0.80
3.58
8.09
4.06
5.00
3.88
6.81
3.71
3.94
4.83
4.71
9.98
5.44
9.06
5.20
6.70
7.35
6.09
4.25
5.29
4.38
5.81
4.37
5.50
4.00
4.69
5.18
7.47
4.15
5.41
162
290
174
273
230
325
174
183
370
235
360
198
287
1080
602
601
680
135
406
192
679
173
258
255
420
244
490
198
903
0.25
2.00
0.50
0.20
2.00
0.50
3.00
2.00
3.00
2.00
3.00
3.00
4.00
4.00
3.00
4.00
4.00
3.00
3.00
5.50
3.00
12.00 0.50
8.00 2.00
40.00 0.20
19.00
8.00
28.00
18.00
17.00
32.00
15.00
11.00
13.00
0.25
32.00
30.00
24.00
42.00
19.00
25.00
25.00
25.00
17.00
27.00
22.00
35.00 3.00
28.00 5.00
17.00 3.50
12.00 3.00
18.00 2.00
0.50
2.00
0.20
0.25
2.00
0.50
0.20
2.00
0.50
3.00
2.00
3.00
2.00
3.00
3.00
4.00
4.00
3.00
4.00
4.00
3.00
3.00
5.50
3.00
3.00
5.00
3.50
3.00
2.00
0.70
6.60
6.40
3.00
6.20
0.25
4.40
0.25
2.60
3.40
3.10
7.00
0.25
0.25
0.25
1.20
2.80
3.40
4.20
0.25
4.90
3.20
2.80
1.40
2.00
3.50
1.10 6.10
0.25 0.25
2.60 18.50
4.70
1.30
1.10
1.30
0.25
0.25
1.00
0.25
1.70
1.50
0.25
1.50
0.25
0.25
0.25
1.10
1.30
2.80
2.80
0.25
0.50
1.30
2.70
4.50
1.40
0.80
Th
Sm
Gd
U
B
Cl/Cl Na2O MgO Al2O3 SiO2 P2O5 K2O CaO TiO2 MnO Fe2O3 Cl
wt. % wt. % wt. % wt. % wt. % wt. % wt. % wt. % wt. % wt. % ppm ppm ppm ppm ppm ppm
36
(Continued)
Cl Age ±
1s c
36
Appendix
in the northeastern Cascade Range, Washington
163
b
c
P-2
P-3
P-4
P-5
P-6
P-7
P-8
P-9
P-10
P-11
P-12
P-13
47.54
47.54
47.54
47.54
47.54
47.54
47.54
47.54
47.54
47.54
47.54
47.54
50.3
50.3
50.3
50.3
50.3
50.3
50.3
50.3
50.3
50.3
50.3
50.3
120.68
120.68
120.68
120.68
120.68
120.68
120.67
120.67
120.67
120.68
120.68
120.68
741
732
756
741
732
701
777
768
768
768
738
738
Sample Latitude
Eff.
Longitude Altitude
Latitude
m
Number
°N
°W
(Map)
°Na
2.7
2.7
2.7
2.7
2.7
2.7
2.7
2.7
2.7
2.7
2.7
2.7
1.0
1.0
1.0
1.0
2.0
2.0
2.0
2.0
2.0
1.0
1.0
1.0
0.96
0.96
0.94
0.94
0.96
0.97
0.97
0.96
0.97
0.97
0.93
0.94
Densityb Thickness Shielding
gm/cm3
cm
All
effects
104.3 ± 3.5
104.3 ± 1.7
93.7 ± 1.6
103.8 ± 1.6
108.9 ± 1.5
108.0 ± 1.6
106.6 ± 1.7
112.8 ± 1.7
102.5 ± 1.5
104.9 ± 1.5
93.0 ± 1.5
112.7 ± 1.7
Cl Age ±
1sc
36
707
1009
756
1009
725
678
789
871
791
668
763
807
3.84
4.16
4.04
4.01
4.05
3.98
3.89
4.18
4.05
3.85
4.24
4.23
4.89
2.51
2.88
2.65
3.34
3.93
3.41
3.54
3.12
5.42
3.30
3.25
16.50
15.60
15.30
15.60
16.00
15.80
15.50
15.90
15.70
16.30
16.00
16.10
56.80
64.70
63.90
64.50
62.50
61.70
63.40
63.20
63.60
57.70
62.60
62.50
0.07
0.06
0.50
0.13
0.11
0.11
0.31
0.07
0.12
0.19
0.09
0.08
0.82
1.65
1.80
2.04
1.49
1.53
1.56
1.06
1.61
0.93
1.14
1.12
6.83
4.76
4.90
4.43
5.46
5.75
4.97
5.37
4.69
6.81
5.42
5.40
0.67
0.47
0.50
0.52
0.54
0.63
0.59
0.58
0.67
0.69
0.61
0.58
0.10
0.06
0.08
0.07
0.09
0.09
0.08
0.08
0.08
0.11
0.08
0.02
6.46
3.74
4.55
4.03
5.43
5.50
5.35
4.97
4.95
6.77
5.01
5.03
422
169
300
208
454
430
352
236
350
415
266
576
19.50
16.00
17.00
11.00
34.00
28.00
24.00
20.00
22.00
23.00
12.00
12.00
5.00
2.00
3.00
0.05
0.25
3.00
0.25
0.25
0.50
3.00
2.00
2.00
5.00
2.00
3.00
0.05
0.25
3.00
0.25
4.00
0.50
3.00
2.00
2.00
1.30 4.60
1.10 7.80
1.80 18.60
0.25 0.25
1.30 3.80
0.60 4.10
1.10 1.60
0.25 3.20
1.10 0.25
0.25 2.50
0.25 3.30
1.00 1.00
Cl/Cl Na2O MgO Al2O3 SiO2 P2O5 K2O CaO TiO2 MnO Fe2O3 Cl
B
Sm
U
Th
Gd
wt. % wt. % wt. % wt. % wt. % wt. % wt. % wt. % wt. % wt. % ppm ppm ppm ppm ppm ppm
36
Calculated effective latitude is integrated over the exposure age of the respective sample
Assumed rock density for granodiorite boulders is 2.7 gm/cm3
Ages are reported with 1␴ AMS uncertainty; all cited 36Cl ages are rounded to the nearest 100 years; values in bold type are outliers.
a
PESH 1-2
PESH 1-3
PESH 1-4
PESH 1-5
PESH 1-6
PESH 1-7
PESH 1-8
PESH 1-9
PESH 1-10
PESH 1-11
PESH 2-1A
PESH 2-1B
Sample
Number
(Field)
Appendix
(Continued)
164
S. C. Porter and T. W. Swanson—36Cl dating of the classic Pleistocene glacial record
in the northeastern Cascade Range, Washington
165
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